A solar cell able to convert solar energy into electricity is described, based on a new operative principle capable of obtaining efficiency superior to that obtained with the present solar cells. Therefore, this invention pertains to the scope of manufacturing semiconductor devices for electronic use and, more specifically, to the manufacture of solar cells.
The cell, as described in FIG. 1, contains a semiconductor (1) with an intermediate band (2) half filled with electrons, located between two ordinary, n type (3) and p type (4), semiconductor layers. On lighting up electron-hole pairs are created, either by absorption of a photon with the required energy (5) or by the absorption of two (6,7) with less energy that pump an electron from the valence band to the intermediate band (8) and from this to the conduction band (9). A flow of electrons exits mainly through the n type (3) semiconductor and a flow of holes through the p type (4); thus an electric current that exits through the p side and returns through the n side is established. The n type (3) and p type (4) layers also prevent the intermediate band (2) from coming into contact with the external metallic connections, which would cause a short-circuit.
Solar cells, as they are today, are manufactured with a semiconductor and are based on the following operative principle: The photons of the light, on falling on the solar cell, are absorbed by it and yield their energy to the valence electrons of the semiconductor and tear them from the bonds that maintain them joined to the cores of the atoms, promoting them to a superior energetic state called conduction band in which they can move easily through the semiconductor. At the same time, the holes left by the yielded electrons can jump from core to core, thus forming a second type of charge carrier, this time positive, that located in the valence band can also move easily.
In this way, each photon usefully absorbed causes or generates an electron-hole pair. There are a series of mechanisms through which a broken bond can be reconstructed, resulting in what is called a recombination of an electron-hole pair.
Operation of the solar cells can be explained using the following balance arguments. Thus, in the case of the conduction band, the difference between the number of electrons generated by time unit less the number of those recombined must equal the electrons going out of said band as currentxe2x80x94incoming if what is outgoing are electronsxe2x80x94from an external circuit. Likewise, the difference between generations and recombinations equals the holes that go out of the valence band as current, in this case outgoing current.
So that the solar cell operates properly, one of the faces must be made in such a way that nearly all of what leaves it are electrons and the other so that nearly all of what leaves it are holes. This is obtained by the procedure of doping the semiconductor next to one of the faces of the solar cell with donnor impurities, able to produce electrons in the conduction band, so that there is a high concentration of electrons in this regionxe2x80x94n type regionxe2x80x94whilst the semiconductor next to the other face is doped with acceptor impurities, able to produce holes in the valence band, so that there is a high concentration of holes in that regionxe2x80x94p type region.
The abundance of majority carriers, electrons in the n region and holes in the p, reduces that of minority carriers, holes in the n region and electrons in the p. In this way, almost only electrons can leave through the n region as there are hardly any holes, whilst only holes can leave through the p region as there are hardly any electrons. Therefore, with the solar cell illuminated, electrons leave through its n face and holes through its p face or, what is the same, a current leaves through the p face and enters through the n face.
A very important concept in the solids is the Fermi level. This is an energetic level at which the quantum states contain an electron at absolute zero. Above this level the states are empty. At temperatures different from absolute zero, in a semiconductor that is not excessively doped, the following important ratios are fulfilled:
n=Ncexp[(EFxe2x88x92Ec)/kT]xe2x80x83xe2x80x83(E1)
p=Nvexp[(Evxe2x88x92EF)/kT]xe2x80x83xe2x80x83(E2)
where n and p are the concentration of electrons and holes, respectively, in the semiconductor, NC and NV are characteristic constants of the used semiconductor, depending on the temperature, k is the Boltzmann constant, T is the absolute temperature of the semiconductor, EC and EV are the energetic levels of the minimum of the conduction band and the maximum of the valence band, respectively, and, lastly, EF is the Fermi level. It should be noted that the np product, at a given temperature, is a constant called ni2. Use has been made of this fact on indicating that the regions with many electrons have few holes and vice versa.
These equations reveal that the Fermi level is found near the end of the conduction band in n type semiconductors with many electrons and near the top of the valence band in p type semiconductors with many holes.
On the other hand, the Fermi level has a very important thermodynamic meaning as it represents the chemical potential of the electrons in the solid, with the current proportional to its gradient. As regards a semiconductor in thermal equilibrium, in which there are no currents, the Fermi level must have a zero gradient, that is, it must be constant throughout the semiconductor.
Under illumination, the electrons and holes are more abundant and the np product becomes greater than in balance. In this case, the Fermi level is separated into two Fermi pseudo-levels, one, EFn, for the electrons and the other, EFp, for the holes. Using the previous formulas it is easy to see that:
np=ni2exp[(EFnxe2x88x92EFp)/kT]xe2x80x83xe2x80x83(E3)
The separation of the Fermi pseudo-levels, dragging with them the band in which the carrier to which they refer is majority, causes a noticeable modification in the potential difference and, therefore, in the electric field which noticeably decreases. This potential variation is reflected in the potential difference that appears in the terminals of the solar cell, given by the difference of the Fermi pseudo-levels of the electrons on face n and the holes on face p, with the sign changed, in other words, face p becomes positive with respect to face n.
The main parts of a solar cell are: the emitter or upper n region, the base or lower p region, the rear metallic contact and the upper metallic contact in the form of a grid to let the light through.
An inconvenience of the solar cells is that they do not completely convert the energy of the photons they receive into electric energy. In fact, to start, the solar cell can only make use of the photons with energy exceeding that of the forbidden band EG=ECxe2x88x92EV, or the distance that separates the energy minimum of the conduction band from the maximum energy level of the valence band. For photons with more energy, they also fail to the best use of the excess energy provided by the photon. In conclusion, the photons made the best use of are those with energy near EG. For this reason, solar cells in tandem are used.
The tandem cells are made up of two or more cells with different EG values placed one on top of the other in a descending EG so that the one with more EG is on the top. These cells are connected in series, usually by a tunnel junction that provides a good ohmic contact among them. These junctions are such that, due to their high doping, form such a high electric field that the semiconductor is perforated electrically, resulting in a good current conduction.
The efficiency with tandem cells is potentially greater than with cells of one semiconductor only, and it is in practice when the tandem is sufficiently developed.
This invention consists in using a semiconductor with an intermediate band as the source of a new type of cells the efficiency of which exceeds not only the conventional solar cells but also the tandem cells of two semiconductors, all of which are considered as ideal devices.